This invention relates to a method and a punch/die assembly for use in the production of heat exchanger fins as in, for example, air conditioners for rooms and motor vehicles, and, in particular, to a method which does not require any preliminary punching prior to the regular punching indispensable to the conventional manufacturing process of said fins, which contributes to remarkable improvement of productive efficiency and quality of products.
In general, the cooling fins are constituted by a number of superposed square sheets each having a multiple number of flanged apertures in registration with similar flanged apertures in the overlying and underlying sheets, and a number of copper tubes passed through the respective flanged apertures. The flanges are usually turned outwardly to form flares around the marginal edges of the apertures for the purpose of maintaining a predetermined distance between the adjacent apertures and at the same time for reinforcing purposes. Therefore, the flanges are required to have projecting height of at least of, for example, more than 1.8 mm.
In forming a flanged aperture in an aluminum sheet, it has been the conventional practice to perforate or pierce in the first step an aperture which has a diameter far smaller than that of a flanged aperture to be ultimately formed, and then pressing or burring the marginal edge portions upwardly by means of a punch thereby to form the flanged aperture of the predetermined dimension. However, where the aperture is decreased in diameter in an attempt to increase the height of the ultimate flange, cracking often occurs to the marginal edge portions of the aperture when pressed by the punch. Therefore, without a preliminary treatment or machining, it has been difficult to form a flange which has a height greater than 1.8 mm.
The pre-machining usually includes pressing of an aluminum sheet by a punch to form a bonnet-like recess of a diameter far larger than that of the intended flanged aperture and further pressing of the recessed portion by another punch to reduce its diameter while increasing its height. These operations are repeated and then the aforementioned steps are employed to obtain a number of flanged apertures of the predetermined diameter and height. This method is generally referred to as "drawing" and is capable of forming a flange of a relatively great height by the gradual or progressive stretching of the aluminum sheet. However, the just-mentioned method has inherent drawbacks in that the circumferential wall of the flanged aperture bears concentric hammered marks as a result of the repeated punching operation and wrinkles appear at the both sides of the aluminum sheet to cause warping or distortion to the fins as a whole.